A typical horizontal-axis wind turbine is illustrated in FIG. 1. The wind turbine 1 comprises a tower 2, a nacelle 3 mounted at top of the tower 2 and a rotor 4 operatively coupled to a generator 5 within the nacelle 3. The wind turbine 1 converts kinetic energy of the wind into electrical energy. In addition to the generator 5, the nacelle 3 houses the various components required to convert the wind energy into electrical energy and also the various components required to operate and optimize the performance of the wind turbine 1. The tower 2 supports the load presented by the nacelle 3, the rotor 4 and other wind turbine components within the nacelle 3.
The rotor 4 includes a central hub 6 and three elongate rotor blades 7a, 7b, 7c of approximately planar configuration which extend radially outward from the central hub 6. In operation, the blades 7a, 7b, 7c are configured to interact with the passing air flow to produce lift that causes the central hub 6 to rotate about its longitudinal axis. Wind exceeding a minimum level will activate the rotor 4 and allow it to rotate within a plane substantially perpendicular to the direction of the wind. The rotation is converted to electric power by the generator 5 and is usually supplied to the utility grid.
A known rotor blade for such a wind turbine comprises a hollow spar which serves to transfer loads from the rotor blade to the hub of the wind turbine. Such loads include tensile stresses directed along the length of the blade arising from the circular motion of the blade and stresses arising from the wind which are directed along the thickness of the blade, i.e. from the windward side of the blade to the leeward side. In addition, gravitational stresses arise within the blade in approximately the edgewise (or chordwise) direction, i.e. directed from the leading edge of the rotating blade to the trailing edge, when the blade is directed away from the vertical, and these are most pronounced when the blade is horizontal. When the leading edge is above the trailing edge, these stresses are tensile in the leading edge, and when the leading edge is below the trailing edge, the stresses are compressive in the leading edge.
Finally, in a plane approximately perpendicular to the blade chordwise axis, aerodynamic loads are generated across the blade surface due to wind interaction which result in bending loads being directed in the opposing flatwise faces of the blade. The sense of the loads is such that, under steady wind loads, the windward blade faces is generally in tension and the leeward blade faced is generally in compression.
Throughout the present specification, the term “leading edge” refers to the edge of the rotor blade which hits the air during rotation of the blade, and the term “trailing edge” refers to the opposite edge.
The spar may be formed as a single integral structure by a winding a length of fibre around a mandrel. Alternatively, the spar may be formed from two or more, e.g. four, webs which are joined together to form the hollow structure. In order to mitigate the problem of the gravitational stresses mentioned above, an additional elongate web may be attached to the spar so as to increase the stiffness of the rotor blade in the chordwise direction, and thereby improve the ability of the rotor blade to transfer load in the chordwise direction. A first end of the additional web is attached to the root end of the blade, i.e. the end which will be attached to the hub of the wind turbine. The additional web extends away from the spar within the trailing edge of the rotor blade, and is optimally placed in the chordwise sense to provide both increased stiffness to the rotor blade in the chordwise direction and to provide support against buckling for the windward and leeward face panels by reducing their effective spans.
The elongate additional web also serves to increase the stiffness of the rotor blade along its longitudinal axis.
The spar, or each individual web of the spar, is typically manufactured by winding glass fibre or other suitable fibre around a mandrel. One problem which arises is how to manufacture a spar having an external surface which allows the additional web to be attached at the desired angle. This not only requires the attachment surface to have the desired orientation relative to the longitudinal axis of the spar but also requires the attachment surface to be of an area sufficient to prevent the additional web from becoming detached from the spar in use, when it is subjected to the severe stresses mentioned above.
It would be possible to form the desired surface on the spar by a complex winding operation which involves the insertion of one or more suitable formers during the winding process such that the subsequent windings form a surface having the desired configuration.
However, such a process is time-consuming, since the winding operation must be interrupted to permit the formers to be inserted. Furthermore, it is not a straightforward matter to form certain surfaces, such as concave surfaces using a winding process.
It would therefore be desirable to provide a method of manufacturing a wind turbine blade which overcomes, or at least mitigates, the above disadvantages.